Planning out the mold. Using legos, a box is made as the mold enclosure. The parts are placed into the box to plan out the eventual layout. Having a well planned out layout will reduce the amount of silicon rubber needed; reducing waste.
First part of the mold:
The mold I am making here will be a two part mold, so the original parts are removed from the box and a thin layer of playdoh is placed into the box and flattened. The parts are placed into the playdoh and pressed down slightly to secure them. Once the parts are placed into the box, the empty spaces are filled with cut up pieces of silicon from old molds that I have from previous casts. This is just filler to help reduce the amount of silicon to pour. Silicon bonds perfectly with itself, so the filler material will just become part of the mold once the poured silicon cures.
With this set, I can mix the silicon and pour it. The silicon I’m using is alumilite high strength 2 set. The silicon is mixed by weight, so using a digital scale, I measure out the proper mix ratio according to the instructions. The silicon is mixed with the activator until it is a consistent color. Once mixed, the silicon rubber is poured into the mold. I’m not too worried about bubbles right now as I’ll be placing the uncured mold into a pressure pot for the curing process. In the following picture, you can see that there are a few bubbles on the surface. The mold is then placed into a pressure pot. My compressor is connected to the pressure pot and a constant pressure of 40 psi is applied and the mold is allowed to cure over night, or according to the instructions.
Once cured, the mold is removed from the pressure pot. The bubbles in the mold while it was uncured should have been removed with the process of pressurizing the mold making process. With the mold cured, the legos are removed, followed by removing the playdoh. The playdoh is washed off and the bottom part of the mold is dried completely. A few holes are drilled and brass rods are inserted into these holes.
Second part of the mold:
The mold is now ready for making the second half of the two part mold. The lego box is rebuilt and the mold is now placed into the box. Since silicon will bond perfectly to itself, to ensure that the mold will be in two distinct pieces, a layer of Vaseline is used to keep the two pieces of the mold from sticking. With the Vaseline applied, a new mixture of silicon is poured and the mold is returned to the pressure pot for curing.
After a full day of curing, the mold is ready to be separated. Once cured, the original parts can be removed from the bottom half of the mold. The parts are carefully removed from the mold, and in some instances, the rubber needs to be cut so that the original parts can be easily removed.
Mold prep for casting:
With the original parts removed, a mold release is sprayed onto the two halves of the mold and allowed to dry. A blow dryer is used to heat up the mold. This works to activate the mold release as well as expands the mold slightly for the casting process.
The resin is mixed by weight, 1:1. A clean disposable cup is best. The resin starts to activate very quickly, so after mixing the two parts of resin for a few seconds to make sure it is fully incorporated; the resin is then slowly poured into the bottom mold. Excess resin is poured on top and then the top piece of the mold is placed onto the mold. A weight is placed on top of the combined two part mold and then the entire assembly is placed into the pressure pot.
The resin will fully set up after a couple of hours. For a solid cure, the mold is left in the pressure pot over night. However, the parts can be removed from the mold after about an hour of setting – the parts will just be a bit pliable. With the mold removed from the pot, the top piece of the mold is removed. The resin casted parts are then carefully removed from the mold. Once removed the parts are allowed to sit for another few days to fully cure and harden. Again, with only an hour of setting up, the parts will still be soft and pliable until allowed to fully cure.